Phase shifting design and layout for static random access memory

ABSTRACT

Methods and apparatuses for fully defining static random access memory (SRAM) using phase shifting layouts are described. The approach includes identifying that a layout includes SRAM cells and defining phase shifting regions in a mask description to fully define the SRAM cells. The phase conflicts between adjacent phase shifters are resolved by selecting cutting patterns designed for the SRAM shape and functional structure. Additionally, the transistor gates of the SRAM cells can be reduced in size relative to the original SRAM layout design. Thus, an SRAM cell can be lithographically printed with small, consistent critical dimensions including extremely small gate lengths resulting in higher yields and improved performance.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to manufacturing small dimension features of objects, such as integrated circuits, using photolithographic masks. More particularly, the present invention relates to phase shift masking of complex layouts for integrated circuits and similar objects.

[0003] 2. Description of Related Art

[0004] Phase shift masking has been applied to create small dimension features in integrated circuits. Typically the features have been limited to selected elements of the design, which have a small, critical dimension. See, for example, U.S. Pat. No. 5,766,806.

[0005] Although manufacturing of small dimension features in integrated circuits has resulted in improved speed and performance, it is desirable to apply phase shift masking more extensively in the manufacturing of such devices. However, the extension of phase shift masking to more complex designs results in a large increase in the complexity of the mask layout problem. For example, when laying out phase shift windows on dense designs, phase conflicts will occur. One type of phase conflict is a location in the layout at which two phase shift windows having the same phase are laid out in proximity to a feature to be exposed by the masks, such as by overlapping of the phase shift windows intended for implementation of adjacent lines in the exposure pattern. If the phase shift windows have the same phase, then they do not result in the optical interference necessary to create the desired feature. Thus, it is necessary to prevent inadvertent layout of phase shift windows in phase conflict near features to be formed in the layer defined by the mask.

[0006] In the design of a single integrated circuit, millions of features may be laid out. The burden on data processing resources for iterative operations over such large numbers of features can be huge, and in some cases makes the iterative operation impractical. The layout of phase shift windows and the assignment of phase shift values to such windows, for circuits in which a significant amount of the layout is accomplished by phase shifting, is one such iterative operation which has been impractical using prior art techniques.

[0007] Phase shifting layouts for memory cells have been developed that phase shift gate portions of the memory design for improved performance.

[0008] Because of these and other complexities, implementation of a phase shift masking technology for complex designs will require improvements in the approach to the design of phase shift masks.

SUMMARY OF THE INVENTION

[0009] Methods and apparatuses for fully defining static random access memory (SRAM) using phase shifting layouts are described. By producing the SRAM memory using a “full phase” mask, yield can be improved at smaller sizes (relative to using the same lithographic process with a non-phase shifting mask, particularly the wavelength of light, λ), integrated circuit density is improved by tighter packing of smaller memory cells, and also the performance of the memory can be improved.

[0010] The approach includes identifying that a layout includes SRAM cells and defining phase shifting regions in a mask description to fully define the SRAM cells. The identification may include an automated detection of layout patterns that correspond to SRAM cells, parameterized shape detection, user identification of SRAM cells either interactively through a user interface and/or through input parameters, and/or other identification approaches.

[0011] A region around the layout shapes for an SRAM cell can be identified where phase shifters will be placed in the mask definition. By placing shifters in this region, destructive interference of light of opposite phases will cause definition of the pattern. However, it is necessary to break, or cut, the phase windows in the region to fully permit definition of the feature using phase shifters of opposite phases on opposing edges of the layout shapes of the SRAM cell.

[0012] The cuts can be light transmissive phase shifters as well at intermediate phase values (continuous, 90, 60-120) relative to the primary phase shifters (0 and 180).

[0013] The portion of the SRAM memory cell layout that is more difficult to define using phase sifting looks generally like two T's with off-centered bars interlaced with one another. There are contacts at the base of the bars and four transistors on either end of the top of the T. There are two additional transistors disposed above the interlaced T portion.

[0014] Several locations where cuts will be admitted are used by embodiments of the invention: contact to contact, inside corners of the T's to field, back of T's to back of adjacent T's, contacts to field, and corners of T's to contacts. By selecting one or more of these cutting locations a phase shifting layout of the SRAM memory cell is possible.

[0015] Most mask layouts will select a single cutting pattern for all SRAM memory cells in a particular area. For example, the cutting pattern of using the inside corners of the T's to field together with the back of T's to back of adjacent T's for all SRAM memory cells could be used for all of the SRAM memory cells in a given integrated circuit.

[0016] Additionally, attention may be given to ensuring that corresponding features from one SRAM memory cell to another are defined using the same phase ordering. For example if the phase shifter on the left a given transistor is phase 0 and the one on the right is 180, then it may be desirable to ensure that the phase shifter on the left of the corresponding transistor on another SRAM memory cell is 0 and the one on the right is 180. This ensures consistency in the SRAM memory cell layout even if there is a light intensity imbalance between 0 and 180 degree phase shifters.

[0017] Embodiments of the invention can be viewed as methods of manufacturing an integrated circuit. Embodiments of the invention include phase shifting and/or complementary trim masks for use in defining a layer of material in a photolithographic process.

BRIEF DESCRIPTION OF THE FIGURES

[0018] The file of this patent contains at least one drawing executed in color. Copies of this patent with color drawing(s) will be provided by the Patent and Trademark Office upon request and payment of the necessary fee.

[0019]FIG. 1 illustrates a combination T-L junction.

[0020]FIG. 2 illustrates a phase shifting layout for the T-L junction of FIG. 1.

[0021]FIG. 3 illustrates a squared-U layout with the cutting location indicated for the phase layout.

[0022]FIG. 4 illustrates a U layout with the cutting location indicated for the phase layout.

[0023]FIG. 5 illustrates a H shaped features with a variety of spacings and phase shift layout arrangements.

[0024]FIG. 6 is a simulation of the layout of FIG. 5.

[0025]FIG. 7 includes portions of the simulation of FIG. 6 at higher magnification.

[0026]FIG. 8 illustrates a portion of a layout of an static random access memory (SRAM) cell being defined using phase shifting.

[0027] FIGS. 9-12 illustrate different phase layout designs for the SRAM cell of FIG. 8

DETAILED DESCRIPTION

[0028] Overview

[0029] Layouts and arrangements for defining several types of patterns using phase shifting will be considered.

[0030] T-L Junction

[0031]FIG. 1 illustrates a combination T-L junction, specifically the layout of FIG. 1 includes the feature 100 that includes an L shape adjacent to a T shape. A preferred shifter area 102 is shown surrounding the feature 100. The preferred shifter area 102 corresponds to the preferred phase shifter width for use in defining the feature 100 using phase shifting. Further, cutting locations where shifters defined in the preferred shifter area 102 may be placed. Specifically, the cut locations 104, 106, 108, and 110 are identified.

[0032]FIG. 2 illustrates a phase shifting layout for the T-L junction of FIG. 1. More specifically, the cut location 106 was selected—to minimize the number of cuts—and thus four shifters: the shifter 204, the shifter 206, the shifter 208, and the shifter 210, are used to define the feature 100. The phase shifting layout (and corresponding mask) would include only the shifters. Additionally, a complimentary trim mask for use in conjunction with the layout of FIG. 2 can be developed. See, e.g., United States patent application Ser. No. 09/932,239, having inventors Christophe Pierrat, et. al., entitled “Phase Conflict Resolution for Photolithographic Masks”, filed Aug. 17, 2001, and assigned to the assignee of the present application, which is incorporated herein by reference.

[0033] U Shapes

[0034] Turning to FIGS. 3-4, two U-shaped layouts are shown. FIG. 3 illustrates a squared-U layout with the cutting location indicated for the phase layout. FIG. 4 illustrates a U layout with the cutting location indicated for the phase layout. In each, a single cut on the inside of the U will be used to separate the phase shifters (a corresponding cut can be used on the outside of the U).

[0035] Specifically, FIG. 3 includes the feature 300 and the feature 302. A single cut 304 extends from interior of the corner of the feature 300 towards the feature 302 and then runs parallel along the endcap of the feature 302. Similarly, in FIG. 4 the feature 400 and feature 402 comprise the layout pattern and a cut 404 is used in the interior. Additionally, with respect to FIG. 4, in some embodiments a slightly different cut shape is used in the interior bend of the U. Specifically, a corner 410 and a corner 412 are shown interior to the bended U. In some embodiments, the cut is centered about one of the two corners rather than the full length of the interior angled wall of the U.

[0036] H-Shapes

[0037]FIG. 5 includes a test pattern for H-shapes arranged in a 12 wide by 5 high grid. There are twelve different spacings between the vertical bars of the H shown across FIG. 5 and for each spacing, the column shows a possible shifter arrangement for that spacing. For clarity of reference, each H pattern can be referred to by its x-y position, e.g. (1,1) being the upper leftmost H and (12,5) being the bottom rightmost H. Thus, within a row the space between the bars of the H wider for the H (j′,k) as compared to the H (j,k) where j′>j.

[0038] Which option is ultimately selected will depend on the surrounding environment, e.g. the adjacent polygons, as well as the process latitude. For example, the H shapes in the first row (1,1). . . (12,1) are premised on the assumption that it will be possible to make a cut on both sides of the H. In contrast the H shapes in the second row assume only one cut is possible, e.g. H's (1,2). . . (12,2). The remaining rows make no use of cuts on the outside of the H. But, may result in difficult to manufacture masks, e.g. H (5,3) which has a small phase shift area. Other paterns may allow the phase conflict by not using any cuts, H's (1,5). . . (12,5). It should be noted that some of the H patterns in rows two through four lack adequate space to admit the cutting pattern used in the remainder of the row, c.f. H's (1,2). . . (4,2), (1,3). . . (4,3), and (1,4) 5 . . . (4,4) with the remaining H's in those rows.

[0039] In FIG. 6, a simulated aerial image of the test pattern of FIG. 5 is shown. The exposure conditions assumed that the phase shift mask was exposed with a 248 nm wavelength (λ) light, N.A.=0.75, and σ=0.5 and that the trim mask (not shown) was exposed using the same λ and N.A., but with σ=0.5 and three times the dosage. As can be seen from the simulation in most cases the layouts will be correctable with optical proximity correction. However, there will be a resulting impact on process latitude. For example, the H (6,2) and the H (7,2) can likely print however there will be a limited amount of process latitude for the vertical lines. Similar problem, e.g. with H (5,4), but note that other H's in row four are more likely to be correctable with OPC, e.g. H's (8,4). . . (12,4). In contrast note that in row five, the bars of the smaller H's are more easily corrected with OPC across the phase conflict, e.g. H's (1,5). . . (4.5), while in contrast as the length of the phase conflict area increases correction with OPC becomes more difficult or perhaps impossible, e.g. H's (5,5). . . (12,5).

[0040] Turning to FIG. 7, a magnified view of the simulations for one of the columns, j, at separation 0.5 μm, is shown with the magnified images arranged sideways (e.g. top most row on the left, bottom most row on the right). As shown OPC correction can be applied in the OPC cut regions 710 and the phase conflict region 720. In some instances there may be insufficient process latitude to reliably print the feature, e.g. (j,5) may lack sufficient process latitude to reliably print on the wafer even with optical proximity correction.

[0041] SRAM Cutting

[0042]FIG. 8 illustrates a portion of a layout of an SRAM cell 800 being defined using phase shifting. The SRAM cell 800 is representative of common designs and includes a largely repeating pattern including a portion 810 which is surrounded by a heavy, dashed line. FIGS. 9-12 show several possible phase layouts for fully defining the layout of FIG. 8 using phase shifting.

[0043] Turning to FIG. 9, the portion 810 is shown with a phase shifting layout. The orientation of the cross hatching indicates the relative phase. For example, the phase shifter 912 and the phase shifter 914 have opposite phase (X,X+180) as indicated by the different directions of the cross hatching. The features defined by the shifters, e.g. feature 916, are shown for clarity but are not part of the layout itself.

[0044] The cutting arrangement used in FIG. 9 can be described as having a cut in the phase shifters between the two contacts as well as cuts in the inside corners of the T's opposite the cuts between the contacts.

[0045] As can be seen in FIG. 9, the particular cutting arrangement used cause corresponding features, e.g. the feature 916 and the feature 926, to be defined by a different shifter ordering, e.g. (X, X+180) vs. (X+180, X), as seen by the cross hatching on the shifter 912, 914, 922, and 924. This may be undesirable because there can be a light intensity imbalance between for example 0 and 180 degree phase shifters.

[0046] Thus, the same feature may print slightly differently depending on where it fell within the larger pattern of the cell. For example, if the 0 degree phase shifter is slightly more intense than the 180 degree shifter then the light imbalance will tend to move features slightly towards the 180 degree shifter. If the phase ordering is flipped from corresponding feature to corresponding feature then in some cases the features will print slightly to one side and on others slightly to the other side. As such it may be desirable to ensure that the cutting arrangement used to define the individual memory cells of the SRAM is such that the same feature is consistently defined using the same shifter ordering.

[0047] Such an arrangement is shown in FIG. 10. Specifically, the region 810 is shown with a different shifter and cutting arrangement. Here, the corresponding features, e.g. the feature 1016 and the feature 1026, are consistently defined using the same phase orderings, e.g. phase shifters 1012, 1014, 1022, and 1024.

[0048] The cutting arrangement used in FIG. 10 can be described as having a cut in the phase shifters between the two contacts as well as cuts on the back of the T's to the adjacent T-back.

[0049]FIG. 11 shows a variation on the cutting arrangement of FIG. 10 however, like with FIG. 9 the cutting arrangement causes corresponding features to be defined using alternating phase patterns.

[0050] The cutting arrangement used in FIG. 11 can be described as having a cut in the phase shifters from the contact to the field as well as cuts on the back of the T's to the adjacent T-back.

[0051]FIG. 12 shows a cutting arrangement that like FIG. 10 ensures that the corresponding features are defined using the same phase ordering.

[0052] The cutting arrangement used in FIG. 12 can be described as having cuts in the opposing corners of the T to the contact.

[0053] The particular cutting arrangement selected will depend on mask manufacturability concerns, the process design rules, and/or one or more simulations of the cutting arrangement for a given SRAM design and lithographic process.

[0054] It should also be noted that in some memory designs the horizontally disposed 5 bars for contacts of adjacent memory cells are aligned. Specifically instead both a vertical and horizontal offset between bars 812 and bar 814, the adjacent horizontal bars are in vertical alignment. This would impact the cutting patterns by facilitating the use of straight line cuts between adjacent memory cells, c.f. FIG. 10 and FIG. 11 where a diagonal cut is used.

[0055] Representative Alternative Embodiments

[0056] Additionally, although the description has primarily focused on examples of defining a polysilicon, or “poly”, layer within an IC, phase shifting can be used to define other layers of material, e.g. interconnects, metal, etc.

[0057] Although in many instances, an angled cut is shown as a preferred cutting arrangement, from a mask manufacturing perspective 90 degree cuts are more easily manufactured. Accordingly, in some embodiments, to the extent practical cuts at 90 degrees to the feature are selected in preference to other cuts. This works well at outside corners where the angled cut can be modified to a straight line cut.

[0058] Although the cut areas are shown as clear regions in fact a gradual, e.g. continuous, phase transition can be used as can a tri- or quad-tone mask, e.g. 0-90-180 or 0-60120-180, with the middle phase values used in the cut openings.

[0059] Some embodiments of the invention include computer programs for performing the processes of defining the phase shifting layers and/or corresponding trim layers. In one embodiment, the process is implemented using the abraCAD(TM) software produced by Cadabra Design Automation, a Numerical Technologies company, San Jose, Calif. In some embodiments, the computer programs are stored in computer readable media, e.g. CD-ROM, DVD, etc. In other embodiments, the computer programs are embodied in an electromagnetic carrier wave. For example, the electromagnetic carrier wave may include the programs being accessed over a network.

[0060] As used herein, the term optical lithography refers processes that include the use of visible, ultraviolet, deep ultraviolet, extreme ultraviolet, x-ray, e-beam, and other radiation sources for lithography purposes. The masks designs used should be appropriately adapted, e.g. reflective vs. transmissive, etc., to the particular lithographic process.

[0061] Conclusion

[0062] The foregoing description of embodiments of the invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations will be apparent. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application, thereby enabling others to understand the invention for various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims. 

We claim:
 1. A method for producing a computer readable definition of a photolithographic mask that defines a pattern in a layer to be formed using the mask, wherein the pattern includes a plurality of features corresponding to a plurality of static random access memory (SRAM) cells, the method comprising: selecting a cutting pattern from a set of patterns including at least (i) a first cutting pattern comprising a first cut between two contacts of each SRAM cell and a second and third cut, the second and third cut on inside corners of the T of each SRAM cell opposite the first cut, (ii) a second cutting pattern comprising a first cut between two contacts of each SRAM cell and a second cut on the back of the T of each SRAM cell to the back of the T of an adjacent SRAM cell, (iii) a third cutting pattern comprising a first cut and second cut from contacts of the SRAM cell to the field and a third cut on the back of the T of the SRAM cell to the back of the T of an adjacent SRAM, and (iv) a fourth cutting pattern comprising a first cut and a second cut in opposing corners of the T of each SRAM cell to the contact for use in defining the plurality of SRAM cells; and defining a plurality of phase shifters in the computer readable definition of the mask to fully define the plurality of SRAM cells using destructive light interference, wherein the plurality of phase shifters separated by cuts according to the cutting pattern.
 2. The method of claim 1, wherein the plurality of features have a first gate critical dimension, and wherein the defining a plurality of phase shifters further comprises defining the plurality of phase shifters such that the gate critical dimension less then the first gate critical dimension.
 3. The method of claim 1, further comprising defining a computer readable definition of a second mask, the second mask comprising a complementary trim mask for use in conjunction with the mask and for preserving structure defined by the mask and clearing artifacts created by use of the mask.
 4. The method of claim 3, further comprising manufacturing a photolithographic mask using the computer readable definition of the mask and the computer readable definition of the second mask.
 5. The method of claim 1, wherein the pattern corresponds to a pattern for an SRAM memory chip.
 6. A method of manufacturing an integrated circuit product, the method comprising: identifying a pattern comprised of a plurality of features in a description of a layer of material in the integrated circuit, the pattern corresponding to a plurality of SRAM cells; defining a computer readable definition of a first mask, the first mask comprised of a plurality of phase shifters disposed on an opaque field, the phase shifters disposed such that destructive light interference between adjacent phase shifters defines the pattern, wherein the disposition of the phase shifters according to cuts between locations in the pattern corresponding to one or more of contacts to contacts, back of T to back of T, contacts to corners of T's, contacts to field, and corners of T's to field; defining a computer readable definition of a second mask, the second mask including protective opaque regions on a clear field for protecting the pattern defined using the first mask and clearing artifacts created by the first mask; defining a layer of material in the integrated circuit using a mask produced from the computer readable definition of the first mask and the computer readable definition of the second mask.
 7. The method of claim 6, wherein the integrated circuit comprises an SRAM memory chip.
 8. The method of claim 6, wherein the cuts defined such that corresponding structures of different SRAM cells consistently defined using a particular phase ordering.
 9. The method of claim 8, wherein the cuts comprise contact to contact and back of T to back of T cuts.
 10. The method of claim 6, wherein the cuts comprise phase shifting regions of phase different from those used by the plurality of phase shifters.
 11. The method of claim 10, wherein the cut regions have phase 60 and
 120. 12. The method of claim 10, wherein the cut regions have phase
 90. 13. A photolithographic mask for defining a layer of material, the layer of material including a pattern, the pattern corresponding to a plurality of SRAM cells, the photolithographic mask comprising a dark field mask with a plurality of light transmissive phase shifting regions disposed therein, the disposition of the phase shifting regions such that destructive light interference between the light transmissive phase shifting regions defines the plurality of SRAM cells and according to cuts between locations in the pattern corresponding to one or more of contacts to contacts, back of T to back of T, contacts to corners of T's, contacts to field, and corners of T's to field.
 14. The mask of claim 13, wherein the mask for use in defining a layer of material for an SRAM memory chip.
 15. The mask of claim 13, wherein substantially all of the structure of each of the plurality of SRAM cells defined by the destructive interference created by the positioning of the phase shifting regions.
 16. An apparatus for creating computer readable definition of a mask, the apparatus comprising: means for identifying a pattern, the pattern comprising a plurality of SRAM memory cells in a layout; means for defining a phase shifting region around the pattern, and means for dividing the phase shifting region into a plurality of phase shifters for defining the pattern such that the pattern is fully defined using the plurality of phase shifters, and the phase shifters divided by a plurality of cuts between one or more of locations in the pattern corresponding to one or more of contacts to contacts, back of T to back of T, contacts to corners of T's, contacts to field, and corners of T's to field.
 17. The apparatus of claim 16, wherein the mask for use in defining a layer of material for an SRAM memory chip.
 18. The apparatus of claim 16, further comprising means for defining a computer readable definition of a complimentary mask, the complimentary mask for use in conjunction with the mask to protect the pattern defined using the mask and to clear artifacts created by the first mask.
 19. The apparatus of claim 16, wherein the cuts comprise phase shifting regions having intermediate phase angles relative to the plurality of phase shifters.
 20. A method for producing a computer readable definition of a photolithographic mask that defines a pattern in a layer to be formed using the mask, wherein the pattern includes a plurality of features corresponding to a plurality of static random access memory (SRAM) cells, the method comprising defining a plurality of phase shifters in the computer readable definition of the mask to fully define the plurality of SRAM cells using destructive light interference, wherein the plurality of phase shifters separated by cuts comprised of a first cut between two contacts of each SRAM cell and a second and third cut, the second and third cut on inside corners of T of each SRAM cell opposite the first cut.
 21. A method for producing a computer readable definition of a photolithographic mask that defines a pattern in a layer to be formed using the mask, wherein the pattern includes a plurality of features corresponding to a plurality of static random access memory (SRAM) cells, the method comprising defining a plurality of phase shifters in the computer readable definition of the mask to fully define the plurality of SRAM cells using destructive light interference, wherein the plurality of phase shifters separated by cuts comprised of a first cut between two contacts of each SRAM cell and a second cut on back of T of each SRAM cell to back of T of an adjacent SRAM cell.
 22. A method for producing a computer readable definition of a photolithographic mask that defines a pattern in a layer to be formed using the mask, wherein the pattern includes a plurality of features corresponding to a plurality of static random access memory (SRAM) cells, the method comprising defining a plurality of phase shifters in the computer readable definition of the mask to fully define the plurality of SRAM cells using destructive light interference, wherein the plurality of phase shifters separated by cuts comprised of a first cut and second cut from contacts of the SRAM cell to field and a third cut on back of the T of the SRAM cell to back of T of an adjacent SRAM cell.
 23. A method for producing a computer readable definition of a photolithographic mask that defines a pattern in a layer to be formed using the mask, wherein the pattern includes a plurality of features corresponding to a plurality of static random access memory (SRAM) cells, the method comprising defining a plurality of phase shifters in the computer readable definition of the mask to fully define the plurality of SRAM cells using destructive light interference, wherein the plurality of phase shifters separated by cuts comprised of a first cut and a second cut in opposing corners of T of each SRAM cell to contacts.
 24. An electromagnetic waveform comprising a computer program, the computer program for defining a mask layout, the computer program comprising: a first set of instructions for identifying a pattern comprised of a plurality of features in a description of a layer of material in the integrated circuit, the pattern corresponding to a plurality of SRAM cells; a second set of instructions for defining a computer readable definition of a first mask, the first mask comprised of a plurality of phase shifters disposed on an opaque field, the phase shifters disposed such that destructive light interference between adjacent phase shifters defines the pattern, wherein the disposition of the phase shifters according to cuts between locations in the pattern corresponding to one or more of contacts to contacts, back of T to back of T, contacts to corners of T's, contacts to field, and corners of T's to field; a third set of instructions for defining a computer readable definition of a second mask, the second mask including protective opaque regions on a clear field for protecting the pattern defined using the first mask and clearing artifacts created by the first mask; a fourth set of instructions for defining a layer of material in the integrated circuit using a mask produced from the computer readable definition of the first mask and the computer readable definition of the second mask. 